U.S. Pat. No. 8,435,299 describes an expandable osteosynthesis cage. An embodiment described therein concerns an implant designed to be slid or inserted from a posterior direction between the facing faces of two consecutive vertebrae in order to maintain a given distance between them and to restore the stability of the spinal column (e.g., after a failure of the corresponding joint) by fixing the two vertebrae together.
The patent addresses several techniques for restoring a “normal” lumbar lordosis such as, for example, implanting either a graft (which in time fuses the vertebrae together) or a prosthesis (which fixes them together immediately) while still also making it possible in time to achieve fusion between the vertebra. One such prosthesis is a “cage”, which is hollow and rigid with inside/outside intercommunication slots for receiving a bone graft which, via the slots, subsequently fuses with the adjacent vertebrae on either side.
The patent addresses various types of cages including those having two substantially parallel branches connected to a rigid body through which it is possible to turn a worm screw system. The worm screw system then moves a wedge in screw engagement on the screw from an initial position close to the distal ends of the branches towards the body linking the branches together, thereby splaying the two branches apart angularly. It is then possible to insert such a cage of initially flat shape between the vertebrae, and then by turning the drive axis of the wedge, the desired angle between the branches is adjusted or set from a posterior access. Such devices are more complex mechanically and leave a smaller inside volume for the fusion graft. Also, because of their flat shape even though they are better at ensuring a given bearing angle between the vertebrae, they require a passage of the same rectangular section to be prepared to receive them, and that complicates implementation.